In the study on a preamble sequence on a New Radio (NR) Physical Random Access Channel (PRACH) in a 5th-Generation (5G) mobile communication system, in order to further improve the performance of detecting a preamble, and to lower the probability that short preamble sequences collide with each other, a multi-stage (M-stage) preamble sequence has been designed, where an M-stage preamble sequence including M number of preamble sub-sequences is used for Msg1 transmission in a random access. A User Equipment (UE) selects an M-stage preamble sequence from a predefined or network-configured set of M-stage preamble sequences, and transmits it over a network-configured time-frequency resource. The network (a next-Generation Node B (gNB) or a Transmission and Reception Point (TRP)) detects the time-frequency resource respectively for M preamble sub-sequences in the M-stage preamble sequence. The M-stage preamble sequence can be detected correctly only if all of the M preamble sub-sequences are detected correctly.
FIG. 1 is a schematic diagram of an example of a designed M-stage preamble sequence on an NR-PRACH, where M=2, that is, a 2-stage preamble sequence includes two preamble sub-sequences (Preamble-1 and Preamble-2 as illustrated). There are respective Cyclic Prefixes (CPs) of two consecutive preamble sub-sequences (Preamble-1 and Preamble-2), and a Guard Time (GT) interval is reserved at the tail of the 2-stage preamble sequence. The respective preamble sub-sequences are selected separately by the UE to compose the 2-stage preamble sequence, transmitted as Msg1, and detected respectively at the network side. The 2-stage preamble sequence can be detected correctly only if both of the preamble sub-sequences are detected correctly.
For a root sequence on a PRACH, the root sequence is a Zadoff-Chu (ZC) sequence (simply a ZC root sequence), and since 64 preamble sequences of each cell are generated by cyclically shifting (Ncs, i.e., zero-correlation configuration) the ZC root sequence, and the preamble sequence of the UE is selected randomly, or allocated by the gNB, in order to alleviate the preamble sequences of the adjacent cells from interfering with each other, the indexes of ZC root sequences shall be planned correctly. The indexes of the ZC root sequences are planned by allocating them so that different preamble sequences are generated for the adjacent cells using the indexes to thereby avoid the adjacent cell with the same preamble sequence from interfering with each other.
A Zadoff-Chu (ZC) sequence with a root index uth is defined in Equation (1) of:
                                                        x              u                        ⁡                          (              n              )                                =                      e                                          -                j                            ⁢                                                π                  ⁢                                                                          ⁢                                      un                    ⁡                                          (                                              n                        +                        1                                            )                                                                                        N                  ZC                                                                    ,                  0          ≤          n          ≤                                    N              ZC                        -            1                          ,                            (        1        )            
Where NZC represents the length of the ZC sequence, u represents the root index of the ZC sequence, j=sqrt(−1), and n represents the index of a sequence element. A random access preamble is obtained by cyclically shifting the ZC sequence with the root index uth in Equation (2) of:Xu,v(n)=xu((n+Cv)mod NZC),  (2)
Where Cv represents a cyclic shift, Cv=vNcs, and v represents the V-th cyclic shift Ncs in the range of 0,1, . . . ,└NZC/NCS┘, where └.┘ represents rounding off.
A drawback in the prior art lies in that if a plurality of UEs transmits over the same time-frequency resource, then a preamble sequence may not be detected accurately in the existing M-stage preamble sequence solution.